1. Field of the Invention
The present invention relates to a thin film capacitor, capacitor array comprising a plurality of the thin film capacitors connected in parallel to one another, and an electronic component using these. The electronic component is represented particularly by voltage controlled high frequency resonator, voltage controlled high frequency filter, voltage controlled matching circuit chip, voltage controlled antenna duplexer and the like with excellent power handling capability.
2. Description of Related Art
There is a conventionally known thin film capacitor comprising a supporting substrate with electrical insulation, a thin film lower electrode, a thin film dielectric layer and a thin film upper electrode layer successively stacked in this order. In such a thin film capacitor, the lower electrode layer and upper electrode layer are deposited by sputtering, vacuum deposition or the like, and the thin film dielectric layer is deposited by sputtering, the sol-gel process or the like. In addition, a photolithography process as described below is usually used.
First, a conductor layer serving as the lower electrode layer is formed all over the insulative supporting substrate, and then only necessary portions are masked with a photoresist. Thereafter, unnecessary portions are removed by wet or dry etching, and the lower electrode layer is patterned into a predetermined shape. Subsequently, a thin film dielectric layer is deposited all over the supporting substrate, and then, in the same way as the lower electrode layer, unnecessary portions are removed and the thin film dielectric layer is patterned into a predetermined shape. Lastly, a conductor layer serving as the upper electrode layer is deposited all over the surface, and unnecessary portions are removed to form the upper electrode layer into a predetermined shape. In addition, a protective layer and solder terminal portions are formed, which enables surface mounting.
There is also known the so-called variable-capacitance thin film capacitor which employs (BaxSr1-x)yTi1-yO3-z as the material for the thin film dielectric layer, in which a predetermined bias potential is applied between the upper and lower electrode layers so as to vary the dielectric constant of the thin film dielectric layer, thereby varying the capacitance of the thin film capacitor.
In the variable-capacitance thin film capacitor, the dielectric constant is varied by application of DC bias voltage, and as a result, the capacitance is varied. Change in capacitance also occurs in a high frequency range, so that it can be used as a variable-capacitance thin film capacitor at a high frequency range. By utilizing such capacitance change of the variable-capacitance thin film capacitors at a high frequency range, useful electronic components whose frequency characteristics can be varied by application of DC bias voltage can be produced. For example, in a voltage controlled thin film resonator that combines the variable-capacitance thin film capacitor and a thin film inductor, resonant frequency can be varied by application of DC bias voltage. In a voltage controlled thin film bandpass filter combining the variable-capacitance thin film capacitor or a voltage controlled thin film resonator with a thin film inductor and a thin film capacitor, the bandpass range can be varied by application of DC bias voltage. In addition, variable-capacitance thin film capacitor can be utilized for voltage controlled electronic components for microwaves.
When a thin film capacitor is used as an electronic component, suppressing insertion loss in the high frequency circuit is required. In order to design a high frequency circuit with small insertion loss, a thin film capacitor with high Q-value is required. Q-value of a capacitor can be expressed by the following equation (a).Q=1/(2πfRC+tan δ)  (a)
Here, f represents frequency (Hz), R represents electrode resistance (Ω), C represents capacitance value (F), and tanδ represents dielectric loss of the dielectric. The dielectric loss tan δ is a material constant determined depending on the material for the dielectric, and frequency f (Hz) and capacitance value C(F) are determined by the values designed for the high frequency circuit. Therefore, from equation (a), it is found that when the electrode resistance R becomes high, the Q-value of the thin film capacitor decreases.
In addition, in order to use the thin film capacitor as an electronic component, miniaturization of the component is required at the same time.
Meanwhile, in the manufacture of thin film capacitors, generally, layers having other functions such as a protective layer and a solder diffusion barrier layer are successively stacked in addition to the lower electrode layer, thin film dielectric layer and the upper electrode layer. However, as the number of layers increases, besides problems such as misalignment in the photolithography process and damage to the lower layers during etching, stress is enhanced by the increase in number of the layers, resulting in cracking in the films, which leads to poor characteristics and degraded reliability.
Furthermore, when the capacitance value C is increased by expanding the area in plan view of the thin film dielectric layer without changing the shapes of the lower electrode layer and upper electrode layer, the electrode resistance R does not decrease despite the increase of the capacitance value C. Therefore, from equation (a), Q-value of the thin film capacitor decreases.
It is an object of the present invention to provide a thin film capacitor with small electrode resistance and great Q-value which is compactly designed so as to be applicable to electronic components, and has a decreased number of thin layers that are successively deposited so as to be effective for miniaturization and high-density packaging of the device, and prevents poor characteristics and degradation of reliability.
It is another object of the present invention to provide a variable-capacitance thin film capacitor whose capacitance can be changed by application of bias voltage, which has small electrode resistance and great Q-value and is compactly designed so as to be applicable to electronic components as described above.
It is still another object of the present invention to provide an electronic component using the thin film capacitor such as voltage controlled high frequency thin film resonator, voltage controlled high frequency thin film filter, voltage controlled matching circuit chip, and voltage controlled thin film antenna duplexer which cause little intermodulation distortion and have excellent power handling capability and good temperature characteristics with use of the foregoing thin film capacitor.